File: autoconf.info,  Node: Versioning,  Next: Notices,  Prev: Initializing configure,  Up: Setup

4.2 Dealing with Autoconf versions
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The following optional macros can be used to help choose the minimum
version of Autoconf that can successfully compile a given
‘configure.ac’.

 -- Macro: AC_PREREQ (VERSION)
     Ensure that a recent enough version of Autoconf is being used.  If
     the version of Autoconf being used to create ‘configure’ is earlier
     than VERSION, print an error message to the standard error output
     and exit with failure (exit status is 63).  For example:

          AC_PREREQ([2.72])

     This macro may be used before ‘AC_INIT’.

 -- Macro: AC_AUTOCONF_VERSION
     This macro was introduced in Autoconf 2.62.  It identifies the
     version of Autoconf that is currently parsing the input file, in a
     format suitable for ‘m4_version_compare’ (*note
     m4_version_compare::); in other words, for this release of
     Autoconf, its value is ‘2.72’.  One potential use of this macro is
     for writing conditional fallbacks based on when a feature was added
     to Autoconf, rather than using ‘AC_PREREQ’ to require the newer
     version of Autoconf.  However, remember that the Autoconf
     philosophy favors feature checks over version checks.

     You should not expand this macro directly; use
     ‘m4_defn([AC_AUTOCONF_VERSION])’ instead.  This is because some
     users might have a beta version of Autoconf installed, with
     arbitrary letters included in its version string.  This means it is
     possible for the version string to contain the name of a defined
     macro, such that expanding ‘AC_AUTOCONF_VERSION’ would trigger the
     expansion of that macro during rescanning, and change the version
     string to be different than what you intended to check.